You probably need to create a temporary table, with an IDENTITY column and the PKs of your main table, INSERT the PKs from your Main Table (using an appropriate ORDER BY to get the Identities assigned in the right order), and then JOIN the temporary table to your main table for an Update to set the RNUMBER column to the appropriate new value

It sounds to me like you want to "re-sequence" a tables identity column values on a regular basis. It may be due to my limited imagination but I can't think of a good reason to do this. However depending on what this column is used for I can imagine reasons not to do it. Is this what you're doing? If so...why?